When I started as a teenager, I kept a written diary with the basic details, fees, what I liked/didn't like, etc.
A few years later I started keeping it all on a spreadsheet, which is so much easier. I've added more columns over the years with destinations, clothes, extras, client preferences, things to remember to be more personal, etc. My clients are often pleasantly surprised when I "remember" specific details, favorite activities, or wife/family names that we chat about on dates or pillow talk. It just takes a few seconds to review my notes while getting ready to meet. This is another way to retain favorite clients.
And it's fun to do things like @sum or @counta in Excel to get stats, plus everything is always encrypted for safety. Now with the cloud, I can also see it all from my iPad in the nightstand while still in bed.